The linkage between the four-step binding of oxygen and the binding of heterotropic anionic ligands in hemoglobin

The linkage between the four-step binding of oxygen and the binding of heterotropic anionic ligands in hemoglobin was investigated by accurately measuring and analyzing the oxygen equilibrium curves of human adult hemoglobin in the presence and absence of various concentrations of one or two of the following materials: chloride (Cl-), 2,3-diphosphoglycerate (DPG), and inositol hexaphosphate (IHP). Each equilibrium curve was analyzed according to the Adair equation to evaluate the four-step oxygen equilibrium constants (Adair constants) and the median oxygen pressure. The binding constants of the anions for the molecular species of hemoglobin carrying j oxygen molecules, Hb(O2)j(j=0,1,...,4), were evaluated from the dependences of the Adair constants and the median oxygen pressure on the anion concentration by introducing a model which takes the competitive binding of Cl- and DPG or IHP into account. Assumptions made in the model are: (a) the hemoglobin molecule has two oxygen-linked binding sites for Cl- which are equivalent and independent and (b) no Cl- can be bound to hemoglobin to which DPG or IHP is already bound and vice versa. Thus, we could obtain values for the intrinsic binding constants of Cl- and DPG, i.e., the constants in the absence of other competitive anions. For IHP, only the binding constants and apparent binding constants for Hb and Hb(O2)2 were obtained. Values of the Cl- binding constants and apparent binding constants for DPG and IHP, i.e., the binding constants in the presence of Cl- for Hb and Hb(O2)4, were in reasonable agreement with literature values. From the binding constants we calculated anion binding curves for Hb(O2)j(J=0,1,...,4), the number of anions bound to Hb(O2)J, And the relationship between fractional anion saturation of hemoglobin and fractional oxygen saturation. The numbers of released anions are not uniform with respect to oxygenation step. This non-uniformity is the reason for the changes in the shape of the oxygen equilibrium curve with anion concentration changes and for the non-uniform dependences of the Adair constants on anion concentration, and also results in non-linear relations between anion saturation and oxygen saturation. The anion binding constants and various binding properties of the anions derived from those constants are consistent with those observed by other investigators using different techniques, indicating that the present model describes the oxygen-linked competitive anion binding well.     

Use of dual wavelength spectrophotometry and continuous enzymatic depletion of oxygen for determination of the oxygen binding constants of hemoglobin

A small stopped-flow cuvette was built into a computer-controlled Cary 210 spectrophotometer. The enzymatic depletion of oxygen in solutions of hemoglobin and myoglobin was initiated by flowing the hemeproteins with the enzyme against a solution of the hemeproteins containing the appropriate substrate. The deoxygenation was homogeneous throughout the solution. Oxygen activity was calculated at each instant of time from the fractional saturation of Mb, determined from observations at the Hb/HbO2 isosbestic wavelength. Fractional saturation of Hb was determined from absorbances at the Mb/MbO2 isosbestic wavelength. The spectrophotometer cycled between these two wavelengths during the deoxygenation. The deoxygenation of HbO2 was largely complete in 20-25 min, whereas the deoxygenation of MbO2 was allowed to proceed for about 1 h. This procedure eliminates equilibration of Hb solutions with a gas phase and replaces oxygen electrode readings with spectrophotometric sensing by Mb, providing essentially instantaneous determinations of oxygen activity and hence 250-500 or more independent data points per run. The Mb and Hb data vectors require several manipulations to correct for small relative displacements in time and for small non-isosbestic effects. Detailed consideration of the enzyme kinetics allowed oxygen activities to be determined in regions where Mb is a poor sensor. Studies of HbO2 deoxygenation as a function of wavelength show that the determination of the four Adair constants requires in addition the determination of three spectroscopic parameters. Values of the apparent Adair constants, determined without these spectroscopic parameters, depend strongly on the monitoring wavelength.     

Determination of oxygen saturation and hematocrit of flowing human blood using two different spectrally resolving sensors.

The blood parameters oxygen saturation and hematocrit were determined by two different spectral sensors using reflectance spectra from 550 to 900 nm and partial transmission spectra centered at 660 nm. The spectra were analyzed by the method of partial least squares. One sensor consists of a miniature integrating sphere, while the other was fiber-guided. The results show that the geometry of the sensors and different blood flows do not influence the spectral analysis significantly. Independent of the sensor geometry, both hematocrit and oxygen saturation could be determined with an absolute predicted root mean square error of less than 3%. Furthermore, the analysis showed that hematocrit prediction requires eight wavelength regions and oxygen saturation prediction requires four wavelength regions using reflectance spectroscopy. This implies that if the measurement is restricted to reflectance, a spectrometer is indispensable for determining both blood parameters. Hematocrit determination could be improved using reflectance measurements in combination with transmission.     

Impact of chronic phosalone toxicity on Bohr factor and oxygen equilibrium curves of rat.

The multiple sublethal doses of phosalone induced changes in haemodynamics, maximum absorption spectra, oxygen equilibrium curves (OEC), half saturation tension (P50), degree of interaction (n), pH of blood, Bohr factor and circulatory gases of blood in rat, Rattus norvegicus. The results have obviously shown that increase the activation of pyridoxal phosphate, shift of OEC to right and decrease in the affinity of haemoglobin for oxygen at tissue level.     

A low cost apparatus for the automatic determination of precise oxygen equilibrium curves on red blood cell suspensions

An apparatus for the automatic recording of precise oxygen equilibrium curves on red cell suspensions, as well as on hemoglobin solutions, is described. Dual wavelength determination of fractional oxygen saturation is used, eliminating noise for turbid samples and very small blood samples are needed. An example of data is given for trout blood.     

Equilibrium oxygen binding to human hemoglobin cross-linked between the alpha chains by bis(3,5-dibromosalicyl) fumarate

Oxygen equilibrium curves of human hemoglobin Ao (HbAo) and human hemoglobin cross-linked between the alpha chains (alpha alpha Hb) by bis(3,5-dibromosalicyl) fumarate were measured as a function of pH and chloride or organic phosphate concentration. Compared to HbAo, the oxygen affinity of alpha alpha Hb was lower, cooperativity was maintained, although slightly reduced, and all heterotropic effects were diminished. The major effect of alpha alpha-cross-linking appears to be a reduction of the oxygen affinity of R-state hemoglobin under all conditions. However, while the oxygen affinity of T-state alpha alpha Hb was slightly reduced at physiologic chloride concentration and in the absence of organic phosphates, KT was the same for both hemoglobins in the presence of 2,3-diphosphoglycerate (or high salt) and higher for alpha alpha Hb in the presence of inositol hexaphosphate. The reduced O2 affinity arises from smaller binding constants for both T- and R-state alpha alpha Hb rather than through stabilization of the low affinity conformation. All four Adair constants could be determined for alpha alpha Hb under most conditions, but a3 could not be resolved for HbAo without constraining a4, suggesting that the cross-link stabilizes triply ligated intermediates of hemoglobin.     

Slow unfolding and refolding kinetics of the mesophilic Rop wild-type protein in the transition range.

We describe the guanidinium hydrochloride induced folding kinetics of the four-helix-bundle protein Rop wild-type (wt) under equilibrium conditions at three temperatures. The choice of appropriate denaturant conditions inside the transition range permitted, in combination with equilibrium transition curves, the determination of both unfolding and refolding rate constants. The ratio of the rate constants at zero denaturant concentration provided equilibrium constants and standard free energy changes that are in good agreement with values obtained in previous differential scanning calorimetry studies. The DeltaG0D values for 19, 25 and 40 degrees C calculated from the present kinetic studies are, respectively, 66.8, 70.8 and 57.2 kJ.mol-1. The unfolding reactions are extremely slow under these conditions. Equilibrium was reached only after 18, 12 and 6 days at 19, 25 and 40 degrees C. These results demonstrate that for Rop wt high stability correlates with slow folding kinetics.     

PH dependence of the Adair constants of human hemoglobin. Nonuniform contribution of successive oxygen bindings to the alkaline Bohr effect.

In order to solve the problem of an apparent discrepancy between the pH variance of oxygen equilibrium curve and the linear relation between the number of released Bohr protons and the degree of ligation, precise oxygen equilibrium curves of human hemoglobin were determined at a number of pH values from 6.5 to 8.8. From the equilibrium data individual steps (Adair constants), ki (i equals 1, 2, 3, 4), were obtained and the number of Bohr protons (deltaHi+) released on the ith stage of oxygenation was estimated. The pH dependence of k4 was very small, while the other ks strongly depended on pH over the pH range examined. As a consequence, the contribution of each step of oxygen binding to the alkaline Bohr effect nonuniform: deltaH4 was very small compared with deltaH1+, deltaH2+, and deltaH3+. In spite of this, calcuation has shown that the fractional number of released protons is essentially proportional to fractional oxygen saturation because of cooperative effects in hemoglobin. Thus, the present study indicates that the linear relationship between the fractional number of released protons and the degree of ligation, as obtained from titration experiments, is not necessarily incompatible with the pH variance of the shape of the oxygen equilibrium curve. The nonuniform pH depencence of the Adair constants implies that the two-state allosteric model of Monod, J., Wyman, J., and Changeus, J.P. (1965) J. Mol. Biol. 12, 88-118 is not adequate to describe the heterotropic effect caused by protons.     

A plausible sequence of the conformational changes of hemoglobin induced by oxygenation.

Recent experimental data of oxygen equilibrium constants of human adult hemoglobin, which are measured over a wide range of oxygen pressures, are analyzed successfully from the viewpoint that the change in the molecular structure of hemoglobin induced by oxygenation is considered individually at each stage of oxygenation. Then, a simple phenomenological rule, which explains quantitatively the values of the four Adair constants with only three parameters, is found for hemoglobin under normal physiological conditions. The temperature dependence of these parameters suggests a sequence of the conformational changes such that until the third stage of oxygenation the conformational changes occur within the deoxy quaternary structure and at the fourth stage of oxygenation the deoxy quaternary structure is altered to the oxy one. The effects of pH and phosphate compounds on the Adair constants are discussed, and a possible modification and extension of the rule is suggested. The connection between the rule and the molecular structures of deoxy- and oxyhemoglobin is also discussed.     

Modeling the enzymatic transformation of 3,5-dimethoxy,4-hydroxy cinnamic acid by polyphenoloxidase from the white-rot fungus Trametes versicolor

A mechanism for transforming sinapic acid by a polyphenoloxidase from Trametes versicolor was investigated using changes in sinapic acid and oxygen concentrations during the reaction. The experiments were performed in a closed system without supplemental oxygen. The effects of temperature and initial oxygen concentration on the reaction rates were examined. To compare the obtained results with those from spectrophotometric studies, some runs were performed using an open system with supplemental oxygen. Sinapic acid transformation can be described by the Theorell-Chance Bi-Bi or Ordered Bi-Bi mechanisms. This reacting system consisted also of additional enzymatic reactions between the products of sinapic acid transformation and oxygen. A mathematical model was developed using four ordinary differential equations that represent the Theorell-Chance Bi-Bi mechanism with three alternate substrates. Model parameters (i.e., rate constants) were determined using the data collected at three different temperatures. On the basis of the transition state theory, relationships between these constants and temperature were established. It is shown that, in the open system, the observed change in the enzyme activity at higher temperatures was caused by two opposing phenomena: an Arrhenius effect which increased the rate, and a solubility effect which reduced the rate due to a lower oxygen concentration. This finding allows us to recommend better conditions for spectrophotometric methods, the assay most commonly used to evaluate this and similar enzymes. (c) 1996 John Wiley & Sons, Inc.     

Functional and structural analysis of catalase oxidized by singlet oxygen

Purified catalase-1 (CAT-1) from Neurospora crassa asexual spores is oxidized by singlet oxygen giving rise to active enzyme forms with different electrophoretic mobility. These enzyme forms are detected in vivo under stress conditions and during development at the start of the asexual morphogenetic transitions. CAT-1 heme b is oxidized to heme d by singlet oxygen. Here, we describe functional and structural comparisons of the non-oxidized enzyme with the fully oxidized one. Using a broad H(2)O(2) concentration range (0.01-3.0 M), non-hyperbolic saturation kinetics was found in both enzymes, indicating that kinetic complexity does not arise from heme oxidation. The kinetics was consistent with the existence of two kinds of active sites differing more than 10-times in substrate affinity. Positive cooperativity for one or both of the saturation curves is possible. Kinetic constants obtained at 22 degrees C varied slightly and apparent activation energies for the reaction of both components are not significantly different. Protein fluorescence and circular dicroism of the two enzymes were nearly identical, indicating no gross conformational change with oxidation. Increased sensitivity to inhibition by cyanide indicated a local change at the active site in the oxidized catalase. Oxidized catalase was less resistant to high temperatures, high guanidinium ion concentration, and digestion with subtilisin. It was also less stable than the non-oxidized enzyme at an acid pH. The overall data show that the oxidized enzyme is structurally different from the non-oxidized one, although it conserves most of the remarkable stability and catalytic efficiency of the non-oxidized enzyme. Because the enzyme in the cell can be oxidized under physiological conditions, preservation of functional and structural properties of catalase could have been selected through evolution to assure an active enzyme under oxidative stress conditions.     

Kinetics of the co-operative reaction of Helix pomatia hemocyanin with oxygen. Oxygen binding at low and intermediate oxygen saturations

The kinetics of oxygen binding of Helix pomatia α-hemocyanin has been studied at low and intermediate levels of ligand saturation, under conditions in which oxygen binding is highly co-operative. Temperature-jump relaxation spectra are heterogeneous and can be resolved into a slow and a fast phase. The latter is related to a bimolecular reaction, i.e. the binding of oxygen. At very low degrees of fractional saturation (<0.15) the reactant concentration-dependence of the faster relaxation rate allows the combination and dissociation rate constants of the low affinity or T-state to be estimated as 1.3 × 10⁶m^?1 s^?1 and 300 s^?1, respectively. A possible interpretation of the slow component in the relaxation spectrum is discussed.In stopped-flow experiments, after mixing deoxyhemocyanin with oxygen-containing buffer, most of the binding process to the T-state is lost in the dead time. The observed initial rates of oxygen binding are between 15 and 120 s^?1. depending on the oxygen concentration, and may reflect the rate of the allosteric change from a low to a high affinity state (T→R transition), which is slower than oxygen binding.Similarities and differences in the overall kinetic properties of small and giant respiratory proteins, i.e. hemoglobin and hemocyanin, are discussed.     

Influence of dissolved oxygen concentration on the oxygen kinetics of Gluconobacter oxydans

Summary As shown in earlier studies in production scale bioreactors oxygen limited zones occur. Microorganisms in these reactors are therefore subjected to concentrations of oxygen varying with time. To simulate these conditions, the effect of low oxygen concentrations upon product formation and kinetics of oxygen of Gluconobacter oxydans are studied at laboratory scale.Under these oxygen limited conditions comparable kinetic parameters for oxygen are observed as under normally aerated conditions.So, a saturation constant for oxygen K _{O 2}=6.9 mol/l is observed, which is equivalent to a DOT value of about 3% of air saturation.For optimization purposes of production scale conditions, gassing with oxygen enriched air or with pure oxygen is one of the possibilities.To study the effect of high oxygen concentrations upon kinetics and product formation, the organisms are also cultivated under these extreme conditions. Although at oxygen concentrations larger then 60% saturation with pure oxygen, still growth was observed, the growth rate and also the product formation rate were strongly diminished.From these experiments it can be concluded that gassing with pure oxygen to achieve higher oxygen transfer rates at production scale will be restricted.     

Precision determination and Adair scheme analysis of oxygen equilibrium curves of concentrated hemoglobin solution. A strict examination of Adair constant evaluation methods

To examine the validity of the recent finding by Gill et al. (S.J. Gill, E. Di Cera, M.L. Doyle, G.A. Bishop and C.H. Robert, Biochemistry 26 (1987) 3995) that the third overall Adair constant (A3) for human hemoglobin tetramers (Hb A) is too small to be determined and therefore that the contribution of the triply ligated species in the oxygenation process is negligibly small, highly accurate oxygen equilibrium curves for concentrated pure Hb A solutions were determined with an automatic oxygenation apparatus and analyzed by a least-squares curve-fitting method with various options. The present results indicate that an appropriate choice of weighting for data points is the key to the correct evaluation of the Adair constants and the present experimental data cannot accommodate the Adair scheme with A3 = 0, giving distinctly positive values for A3. Several criteria for correct determination of the Adair constants are presented.     

Studies of oxygen binding energy to hemoglobin molecule.

The fractional oxygen saturation of hemoglobin and of its isolated α-chains have been determined by the application of the technique of Mossbauer effect while the oxygen equilibrium pressure was measured directly. By comparing the experimental data so obtained with theoretically derived oxygen saturation curves for hemoglobin and isolated α-chains the values of the oxygen pairing energy and oxygen binding energy to hemoglobin molecule have been found to be 0.043 ± 0.004 and 0.60 ± 0.06 eV respectively.     

Comparison of the applicability of several allosteric models to the pH and 2,3-bis(phospho)glycerate dependence of oxygen binding by human blood

Oxygen saturation curves of blood were measured by the mixing method at several concentrations of protons and bis(phospho)glycerate. Various theoretical models for the co-operativity of oxygen binding by haemoglobin were then tested for their ability to fit the experimental curves. The effects of pH on oxygen binding could be described by both the Monod, Wyman & Changeux, and the Herzfeld & Stanley models, with most success when protons were assumed to affect oxygen affinity directly rather than through effects on the quaternary-state equilibrium. When the combined effects of pH and bis(phospho)glycerate were considered, however, all the versions of the Monod model that were used, including the three-state version, were unsuccessful. The best fit to the saturation curves was obtained with the Herzfeld & Stanley model, with protons acting as a direct effector of oxygen affinity, and bis(phospho)glycerate acting to lower oxygen affinity as well as influencing the quaternary-state equilibrium.     

On the methodology of dissolved oxygen saturation concentration measurements

Summary An experimental method for the determination of the concentration of dissolved oxygen saturation in real fermentation media is described. It is based on a joint analysis of gas and liquid phases, applying a mass spectrometer for gas analyses and an oxygen electrode for the liquid phase measurements. This method enables the experimental measurement of oxygen solubilities in real fermentation broth within the bioreactor during the process and its application seems to be of general validity.     

Kinetics and mechanism of hydrolysis of acetylthiocholine by butyrylcholine esterase

Kinetics and mechanism of hydrolysis of acetylthiocholine by the enzyme butyrylcholine esterase was studied. The spectrophotometric Ellman's method and potentiometric pH-stat method were used for continuous determination of the actual concentration of the products thiocholine and acetic acid in the reaction mixture. The validity of the Michaelis-Menten (Briggs-Haldane) equation in the whole course of the reaction under used conditions was proved. The corresponding kinetics parameters (Vm and KM) were calculated from the obtained dependences of concentration of thiocholine or acetic acid vs. time and compared. From this comparison the deciding kinetic role of the step producing thiocholine was derived. The values of initial molar concentration of the enzyme and of the rate constants of the kinetic model were estimated.     

Non-linear relationships between oxygen saturation and magnitude of fine structure of ultraviolet oxy versus deoxy difference spectrum in human hemoglobin

Ultraviolet difference spectra of fully oxygenated hemoglobin $\text{vs.}$ successively deoxygenated or reoxygenated hemoglobin were determined in the absence and presence of organic phosphates. Magnitude of fine structure in the difference spectrum around 290 nm, which is considered to be a partial reflection of oxygenation-induced changes in quaternary conformation of hemoglobin, was not linearly related to fractional oxygen saturation of hemoglobin of the reference cell. The non-linear feature was influenced by the organic phosphates as predicted by the allosteric model of Monod $\text{et al.}$ The present study suggests that the ultraviolet oxy $\text{vs.}$ deoxy difference spectrum measurements provide a useful way to examine the validity of the model.